What is bash scripting?

As you seen already, BASH is a “shell programming environment”: a way to tell linux how to run commands. Shell scripts are at the heart of a shell as a programming environment. They allow us to take the commands we repeat frequently and save them in files so that we can re-run all those operations again later by typing a single command.

So how do I make a bash script?

• identify commands you’d like to repeat
• edit a text and insert these commands
  • be sure and add “comments” so you know why you inserted those commands and how to use them
• save the file
• tell bash that you want to “execute” the script (ie. run all those commands)

collect your commands

if I want to write information to the screen, I could use echo :

prompt> msg="TACC Summer Institute"
prompt> echo $msg
TACC Summer Institute

In order to repeat that over and over, let’s create a script using the text editor vi (that we learned earlier):

prompt> vi first_script.sh

let’s enter the above command as well as some “comments”. Lines starting with “#” are not executed, so we can use them to remind us what or why we’re doing something.

# Print a message
STRING="TACC Summer Institute"
echo $STRING

type <ESC>:wq to write the changes then quit vi. for now don’t worry about the variable named “STRING”. A string is a type of variable that holds text data.

“execute” (run) the script

How do we execute a script? Let’s try typing file name:

prompt> first_script.sh
-bash: first_script.sh: command not found
prompt> ./first_script.sh
-bash: ./first_script.sh: Permission denied

OOPS! So those didn’t work because BASH doesn’t implicitly know when to execute commands. Try:

prompt> bash ./first_script.sh
TACC Summer Institute

shebang “#!”

In order to not have to type bash each time, let’s edit the file again and insert a #!(aka shebang) to let bash know what to do (ie. use /bin/bash to run the script)

prompt> vi first_script.sh

#!/bin/bash
# The first line is a "shebang". This line is a comment.
# Print a message
STRING1="TACC Summer Institute"
STRING2="is great!"
echo $STRING1 $STRING2

now bash knows that /bin/bash should execute the commands, but we also need to tell bash it’s allowed to run that script.

prompt> chmod +x first_script.sh

( recall that chmod changes the permissions on a file and +x adds eXecution permission )

prompt> ./first_script.sh
TACC Summer Institute is great!

We can now repeat any given series of commands exactly, allowing for both ease of use and improved reliability.

QUESTION: Why does scripting improve reliability?

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